Monoclonal antibodies against HLA-DR antigens and chicken anti-human Ia serum have been found useful in the characterization of antigens which are expressed in monocytes and endothelial cells and not detectable in B lymphocytes. The possibility that these antigens also are "Ia-like" is being investigated by studying their role, if any, in antigen presentation by monocytes or endothelial cells to T lymphocytes. Pregnancy sera containing such antibodies will be collected in volume of several hundred ml. These sera will be absorbed with platelets to remove antibodies against HLA-A, B, C antigens, and with cultured B cells to eliminate anti-DR reactions, when necessary. The possibility that similar patterns of reactions with monocytes and endothelial cells may be obtained with certain mouse alloantisera will be investigated.